Swedish developments in the bleaching of chemical pulp have increased the interest in extending delignification with oxygen prior to final bleaching, in order to improve the pulp quality of totally chemical free bleached pulp, i.e. TCF-pulp (Totally Chlorine Free), and in order to facilitate the closing of both TCF-systems and ECF-systems (Elementary Chlorine Free).
Several variations of two-stage systems for oxygen delignification are currently in operation. These systems use, for example, extended retention time, re-mixing, and intermediate treatment for chemical distribution, all of which have become possible by use of a two-stage system. Both of these stages are thus carried out in upstream systems, and are pressurized to various degrees.
The following conditions have been found to be particularly advantageous for such delignification processes:
in the first stage the chemical concentration (oxygen and liquor) should be high, i.e. it has been found to be advantageous to charge all of the chemicals to the first stage. In other words, it is important to maintain an initially high pressure in order to support the oxygen reactions. PA1 for extended delignification in the first phase of the oxygen system it is extremely important to maintain a high temperature. If the alkalinity is sufficiently high for a good reaction to take place, no additional chemicals need to be charged to the second stage (the final phase). PA1 no separate pump is required between the reactors; PA1 the pulp is heated at the lowest pressure level in the system, so that low pressure PA1 steam can now be used for heating the pulp; PA1 possible admixture of additional chemicals, such as hydrogen peroxide, and adjustment of the alkali level, can take place before the second stage in a mixer located at the top of the second reactor vessel; PA1 due to the fact that the downstream reactor is not filled with pulp, the system can easily be de-aired (degassed) from the top of the downstream reactor vessel; PA1 the steam recovered by flashing the pulp downstream of the second stage can be used for heating purposes, such as the heating of water; PA1 the temperature of the pulp at the bottom of the downstream reactor can also be lowered by dilution with colder liquid, thereby making it possible to recover the heat which is not flashed off; and PA1 the downstream reactor constitutes a buffer by acting both as a reactor and as a storage tower.
These two-stage systems have the most significant disadvantage of requiring much too high investment costs, but also that only high-quality intermediate pressure steam can be used for heating the pulp between the two stages.